In an analytical instrument that includes an ionization source such as for example electrospray ionization (ESI), an atmospheric pressure gas phase ion separator such as for example a high-field asymmetric waveform ion mobility spectrometer (FAIMS), and a detection system such as for example mass spectrometry (MS), it is advantageous to provide samples to the system in parallel. This allows the analytical instrument to rapidly sequence from measurements made from a first sample delivery system (HPLC for example) and from a second delivery system (preferably, but not necessarily of a type identical to the first). If the detection system is fast compared to the delivery system, then it is advantageous to multiplex a plurality of input streams to the same detector.
Multiplexing a plurality of input streams to the same detector has been attempted using a combination of ESI and MS, but such a combination is generally not very practical because the mass spectrometer is not amenable to having multiple inlets into the vacuum system. At best, the resulting gas flow into each inlet is lower than that of one opening, but more importantly the ion optics system in the vacuum system is not generally designed to accommodate ions coming from more than one ion pathway. Two or more inlets to the same MS is desirable, but is not generally practical.
Commercial systems for permitting two or more electrospray sources to operate in conjunction with one orifice into a mass spectrometer have been described. Most notable of these systems is the Micromass™ system for LockSpray™. Using the LockSpray™ system, a time-of-flight (TOF) mass spectrometer (for example) is re-calibrated intermittently during a measurement by moving a small baffle that temporarily prevents ions from an analytical source of sample from entering the MS vacuum system, whilst permitting ions from a second reference LockMass™ electrospray needle to enter the MS and be detected. Once the calibration using the reference LockMass™ compound is completed, the baffle is returned to its original position to permit the ions from the analytical ESI needle to continue to enter the MS and be measured.
If one inlet to FAIMS is used, all of the existing technology applicable to single orifice mass spectrometers would appear to be applicable. However, since FAIMS operates at atmospheric pressure, ions optionally are introduced via multiple inlets. A version of FAIMS with openings around the circumference of the outer electrode have been described previously, such as for instance in U.S. Pat. No. 6,753,522 which issued on Jun. 22, 2004 in the name of Guevremont et al., the entire contents of which are incorporated herein by reference. The ions originating from one of a plurality of ESI sources are selected by moving a ring version of the curtain plate around the FAIMS such that a single opening in the ring is situated in front of each opening in turn. However, the ring-shaped electrode is mechanically very difficult and inconvenient to actuate in an automated manner. In addition, the ring-shaped electrode does not readily support introduction of ions from different ionization sources via inlet orifices that are disposed at different positions along the length of the FAIMS analyzer.
It would be advantageous to provide a system and method for introducing ions into a FAIMS analyzer that overcomes at least some of the above-mentioned disadvantages of the prior art.